Often, there is a need to convert polyphase AC power into DC power for one or more loads. For example, in an aircraft, provision is often made to convert polyphase 400 Hz AC power supplied by a ground power cart into DC power for on-board loads while the aircraft is on the ground. This AC/DC power conversion may be accomplished by multiple rectifiers connected together in a bridge configuration. However, the currents drawn by such rectifiers are non-sinusoidal and contain high levels of low order harmonics. These harmonic currents can cause high levels of voltage distortion in the waveforms produced by the AC source and hence it has been found necessary to limit the magnitudes of these currents in some fashion.
In the past, AC input harmonic filters have been used to reduce the harmonic currents on the input side of the converter. Alternatively, a rectifier topology employing multiple bridge rectifiers and isolation transformers has been used. This latter approach has been found to be particularly desirable owing to the need to isolate the input of the converter from the output for safety reasons. However, in both approaches, the added components have significantly increased the size and weight of the overall converter, and hence have limited the usefulness of these approaches in applications where size and weight must be kept to a minimum, such as in aircraft.
A different approach is to rectify the polyphase AC power utilizing controllable switching devices instead of uncontrolled diodes. A phase controlled rectifier bridge is one example of a converter that adopts this approach.
A rectifier bridge utilizing a plurality of naturally commutated controlled thyristors, however, can present poor input power factor to the AC source, and hence the AC input source must have a greater capacity than if the bridge could present a unity power factor load thereto.
A further example of an AC/DC converter using controllable switching devices is disclosed in Brewster et al., U.S. Pat. No. 4,143,414. In this patent, three separate single-phase AC/DC converters receive phase-to-phase voltages developed by a three-phase voltage source. Each AC/DC converter includes a first full-wave rectifier which converts the phase-to-phase AC voltage into a DC voltage and an H-bridge converter coupled to the first full-wave rectifier. The H-bridge converter includes first and second pairs of thyristors which are alternately operated and which are coupled to a primary winding of an isolation transformer. A secondary winding of the isolation transformer is coupled to a second full-wave rectifier bridge. The second full-wave rectifier bridges of the AC/DC converters are connected together in parallel to form an output of the overall converter. One disadvantage with the converter shown in the Brewster et al. patent is that the thyristors are not self-commutating and therefore require a resonant commutation circuit for proper operation. This resonant commutation circuit undesirably increases the size and weight of the overall converter. Also, the independent operation of the three converters does not guarantee matching of the supply currents to each, and therefore the triplen harmonics may not cancel perfectly.
A paper by Manias, et al. entitled "Novel Sinewave in AC to DC Converter With High-Frequency Transformer Isolation", appearing in IEEE Transactions on Industrial Electronics, Vol. IE-32, No. 4, November 1985, discloses an AC/DC converter utilizing a cycloconverter connected between an AC power source and a high-frequency transformer and a full bridge rectifier coupled between the high frequency transformer and an output of the converter.
A paper by Manias, et al. entitled "A 3-Phase Inductor Fed SMR Converter With High Frequency Isolation, High Power Density and Improved Power Factor", pp 253-263, copyright 1987, IEEE, discloses a two-stage, inductor fed switch mode rectifier (SMR) topology wherein a three-phase AC power source is coupled to a PWM rectifier, a high frequency inverter, an isolation transformer and a diode rectifier. A phase comparator is responsive to the voltage and current in one of the phases of the AC power source and provides a command signal to a PWM gate control logic circuit which in turn operates switches in the PWM rectifier to control the power factor of the converter and regulate the DC output voltage. The PWM gate control logic circuit also operates switches in the high frequency inverter at a fixed duty cycle of 50%.
A paper by Diego, et al. entitled "A Novel Load Current Control Method for a Leading Power Factor Voltage Source PWM Rectifier", pp 383-388, copyright 1992, IEEE, discloses a PWM voltage source rectifier including an input transformer coupled to an AC source, an input filter coupled to the transformer and a switching rectifier. The switches of the rectifier are controlled in accordance with a fixed PWM pattern based upon the detected DC current and AC voltage to control the power angle and hence the amount of power flow transferred from the AC/DC side.
Other patents and publications disclosing AC/DC converters include: Hombu et al., U.S. Pat. No. 4,599,685; Wilkinson et al., U.S. Pat. No. 4,677,366; Glennon et al., U.S. Pat. No. 4,739,466; Severinsky, U.S. Pat. No. 4,816,982; Williams, U.S. Pat. No. 4,940,929; Severinsky et al., U.S. Pat. No. 4,964,029; Ampo, Japanese Patent Publication 58-179168; Fujii, Japanese Patent Publication 2-36765; and an article by Wernekinck, et al. entitled "A High Frequency AC/DC Converter With Unity Power Factor and Minimum Harmonic Distortion", pp 264-270, copyright 1987, IEEE.